I have a 2003 gmc savana 1500 cargo van and i need heavier duty springs?

the leaf springs on my 1500 gmc savana 4.3ltr v6 seem like they are wore out. the van seems to sag in the rear. i do carry a lot af tools in the back of my van though. i had air shocks on the van at one time. they worked good when it was aired up all the way but they leaked quite a bit. i had to fill them up at least once a week. i had this set up until the shackle that the rear of the leaf springs were connected had tore away from the frame.(i dont know how this happened) when my friend welded them back on he accidentally burned a hole in the air hose for the air shock so he replaced them with gas shocks. now the van it sits really low in the rear and i sc my trailer hitch everywhere. (i tow a small trailer at times.) i was wondering if there is a way to put stiffer leaf springs off of another van,truck, or what ever. i dont want to by new if i dont have to.